Category Archives: Electronics

Electronics

Micro-Tel MSR-902C Receiver: root cause analysis, and a volt meter

Finally, some time to deal with the MSR-902C repairs. After replacing the 7401 TTL, and a 7404 TTL, the band select logic seems to work well, except two bands. This could be traced to a dead transistor on the A3A5 band control board. Still a mystery, what caused all these defects? Tracing the line going to the dead transistor (which appears to be a simple +15 V on/off switch), it only goes to one place – a circuit far inside the receiver. As it turns out, this is a hand-wired circuit, not really a circuit board, but a piece of sheet metal with various solder posts. And, on the other side, two filter. One filter mounted properly, the other tied to it with some thread. As you can see, this holds the filter in place, but it can still move around the other filter – and cause a short on the 15 V rail, including the signal coming from the transistor switch.

902c-moving-filter

902c-filter-short

To avoid similar defects in the future, I put some plastic sheet around the filter, and fixed it in place with better ties.

Finally, time for some alignment of the YIG filter, by using a fairly complex setup, a microwave signal generator, a scope to test the receiver output, etc. – see below picture.

902c-test-setup

The YIG filter needs to be aligned for each band, same for the YTO band edge frequencies. This is all done on the A3B7 board. Not much adjustment needed, fortunately, only some fine tuning of the YIG preselectors.

902c-receiving1

Receiving… quite fun to operate the receiver, easy to tune over the full range of frequencies. Maybe this is what makes it so suitable for detecting microwave bugs.

902c-receiving2

Some last repair relates to the frequency display. It did work in some bands originally, not sure how the defect came about – maybe I slipped with a screwdriver, or some other mishap, or some already damaged part, I can’t tell. But now it only shows erratic values, and without a schematic, it is a tough task to fix it.

902c-voltmeter-board

A fairly complex assembly, keep it mind, it is just a volt meter for the frequency display… so much easier nowadays…

902c-voltmeter

The LED display: hand-wired with Teflon coated wires. Sure, this receiver was never intended for the layman, but for some agencies that don’t care about cost and taxpayers’ money.

902c-handwired-display

After some tests and checks – the voltmeter uses a voltage to frequency/time converter, and a MIC5005 integrated timer! Quite a nice and complex chip for its age!

902c-mic5005c

Two hours later – found the issue. A bad reference diode, 1n821. Unfortunately, no such diode in stock, but it is quite similar to the 1n827, only that the latter is more precise, and more expensive, and only a used part in my bin. But easy to check, just put a resistor in series, and run at about 1 mA, and check the voltage drop over the diode. All good.

902c-1n827

902c-1n827-data

Finally, reception is pretty good over all bands, no detail tests of noise levels done yet, but already now it is clear that this is pretty capable receiver, build with only the best components at a time – just the style is not quite service friendly.

Demodulators work as well, receiving 1 kHz demodulated signal, all looking pretty good and clean.

902c-1-khz-test

Ultra-cheap LED Spot Lights: Failure mode analysis, and some reverse engineering, and some concerns

Something amazing about the advent of LED technology for general lighting is not only the brightness, efficiency, and so on, but also the amazingly low price. Here, 20 light fixtures, including 3 LED elements each, 34 EUR total. That’s a bargain a friend of mine could not resist. But think twice, after about 1 year of occasional usage of these lights – several failed. Brightness is gone, some lightly flashing lights remains.

led-20-pcs-33-eur

Still the price is amazing – considering the price of a singe 1 W LED element, with about 1 EUR retail. Plus the case, heat sink, aluminum circuit board, heat conduction paste, external case, 3 lenses!! No idea how this is made in China, for about 1.5 a piece delivered.

led-1w-led-price

The first suspect – the drivers: each lamp has their own little driver box. Type S3W-0103.

led-driver-case

led-spot-down-light

The parts, and a good quality aluminum board, named CQ-LV8072. This is a universal board, found in many kinds of Chinese LED light fixtures.

led-driver-cq-lv8072-board

Tested the LEDs – turns out, one of the LED elements is dead, and this ruins the whole thing, as all three LEDs are arranged in a series circuit. We can fix this easily by replacing the LED elements, all three, with some good quality elements. Albeit, at almost non-economic cost. Hint – the case and be unscrewed with the heatsink turning vs. the outer case. No need to apply brute force like I did, to open it up.

led-driver-s3w-0103-board

Some reverse engineering reveals a rather simple, but practical circuit. Using S8050 and MJE13003 TO-92 transistors, and a little transformer.

led-driver-s3w-0103-schematic

As you can see, no protection elements, what if the input capacitor shorts out, or if some overvoltage blows the transistor. Could it set your flat on fire? Well, my guess is, yes.

Digital Delay Line: sawtooth corrections of an ultra-precise GPS-reference 1 pps signal, and thermal effects

In an earlier post, I have already introduced the Motorola M12+ timing receiver, which is really a nice and affordable gadget for everyone who needs a precise and accurate time signal. Taking about nanoseconds here. All these timing receiver have something called a sawtooth error, linked to their internal clock. See earlier post: M12 perfect time.

Various methods exist to account for this sawtooth error, first and foremost, correction by software. However, I felt the need for a hardware solution here, to simplify the usage of the 1 pps trigger as a reference signal for phase measurements, and other purposes where the recording of sawtooth correction values would be rather troublesome.
With any such attempt at nanosecond scale, considerable thought needs to be put into the system to avoid introducing any errors larger than those we want to correct. In particular, thermal effects can lead to great long-term jitter, aka, randomly wandering phase.

How can we achieve compensation of the sawtooth error? Well, rather easily, by introducing a variable delay element in the signal chain, and adjusting its delay second by second, to the expected sawtooth error, in ns. Fortunately, the M12+ can be programmed to send out a message, called @@Hn TRAIM Status Msg, which provides, every second, the expected sawtooth error, of the next second. One single command is need to make the M12+ send out this message, very second, from now and forever, until other instructions received, or until the M12+ backup battery is taken out…

See below diagram, a AVR processor is tapping the TxD line, from the GPS receiver, to any host controller or PC (if connected), and whatever messages are send out are checked for the @@Hn message (and @@Ha message, just to display the current time, UTC, and date, on a LCD display connected to the AVR). Note that this works perfectly fine, even when another host, or PC is used to control/read/monitor the M12+. The M12+ uses 3 V logic, but an AVR input can easily handle this as a valid signal, even with the AVR running at 5 V.

dsdelay-rs232-controller2

Glad a processor is doing the decoding work… the GPS messages, a bit too cryptic for me:

gps-messages

Rather than implementing a discrete solution with various delay lines as coax cables, switches, etc, Maxim Integrated provides a marvelous chip, a silicon delay line, DS1023 series, at not marginal, but still acceptable cost, USD 8 per piece.

dsdelay-ds1023-data-sheet

This chip comes in various versions, varying by the delay-per-set, and an 8 bit register, to set the actual delay. Sure, the minimum delay is not “0 ns”, but some odd number, corresponding to the delay of the signal before and after the actual delay line.

dsdelay-data-sheet-2

According to information found in the datasheet, this chip is trimmed for best accuracy, and high thermal stability. Further documents also say that the thermal drift is non-linear, and that no coefficient can be provided. Rather, the delay is specified as an absolute number, over the full temperature range. Well, fair enough, but what does this mean for our present case and actual device under test? With no information available anywhere, it seems, the only way to find out is to measure it. The datasheet maximum error would be a bit more than we want.

dsdelay-data-sheet-3

The schematic is nothing to write home about, a 74F04 is used to buffer the input signal, and a the same F04 is used as an output buffer, providing a nice and fast-rise (or, respectively, fast-fall) 1 pps signal.
The only specialty, a thermistor, and two resistors epoxy-glued to the DS1023-50 top surface! This can be used to heat up the device rather quickly, to 60 degC or more, by providing power from a regulated DC power supply.

dsdelay-schematic

Note the heating element and the thermistor (a rather small, fast response, 100 kOhm NTC) – red frame.

dsdelay-board

The test setup – to measure the temperature effects, is running without the GPS, but with a ~1 kHz fast rise-time pulse, from a HP 8012B pulse generator. Both input and output are connected to a HP 5370B Timer Interval Counter. The latter is a great device, single-shot accuracy of 20~30 ps, if you are into any precision timing tasks, very much worthwhile to get one of these, or a Stanford Research Systems SR620. Time intervals are then recorded as averages of 1k measurement, giving very stable readings with high resolution, certainly to 0.01 ns. For the test purposes, the AVR monitoring the RS232 signal can also be programmed via USB, to set any delay value from 0..255, corresponding to a 0..128 ns delay, plus any baseline delay of the gates and the DS1023-50.

dsdelay-test-setup

dsdelay-5370b-measuring

All connected to a PC via GPIB, and recording the delay values at various settings.

dsdelay-recording

Rather than many words, please inspect these diagrams, which will give you a feeling of the delay and drift to be expected with temperature cycling of the device at various rates (slow cooling, fast heating, slow heating, etc.). These were all recorded at the maximum delay, register set to 0xff, 255. Diagrams show delay, in ns, vs. time, as MJD.

dsdelay-temp-effect

dsdelay-temp-effect2

In absolute numbers, 152.1~152.7 ns variation. Not much. About 1 step. So maybe good enough, and no need to apply any temperature compensation, or to put everything into a thermostated box.

Avantek S081-0321 YIG oscillator: not oscillating at all

One of the best sources of microwave signals still are YIG oscillators/YTO. These do require a good amount of power, magnetic coils, etc, but provide stable and rather low noise output, and good modulation capability. Core element is a small YIG sphere, placed in a magnetic field.

However, for the current unit under investigation (from a 18-26 GHz frontend), type S081-0321, 8.0-13.4 GHz, all the magnetic field and effort is wasted – no output detectable at all, not even a faint signal (checked with various equipment). Knocking it with a (small!) hammer, no effect. Varying the coil current – no effect.
Current consumption on the 15 V rail is normal.

yig-test-no-signal

yig-s081-0321-defective

Well, with all the basics checked, what to do with such hermetically sealed unit, other than using it to satisfy my curiosity about its internals. Hope to trace the defect to some specific part.

But before we consider more destructive measures, let’s try to re-tune the YIG by slightly adjusting the YIG sphere. This is possibly throught the side opening, which is usually welded shut, but can be drilled up rather easily.

yig-adjustment-open

Still no luck, no signal, even after turning the YIG quite a bit.

To look inside, carefully removed the top weld seam on a lathe, and the you can pry the case open.

yig-osc1

What you can see is pretty straightforward, despite all the gold wires. There is an input voltage regulator, from +15 V rail, down to 8 volts (measured about 8.15 V), this is then distributed to the 4 active parts via resistors (the bluish elements). Voltage at the resistors is about 4.3 V, so all stages seem to be adequately powered and current flowing as usual. Still no signal. Also probed other parts of the circuit, with a thin wire, under the microscope. No obvious defect. The gold wires and contact point reveal a good amount of adjustment done by placing/removing bond wires as need to adjust bias currents, probably also frequency response, etc.

yig-osc-closeup

The coil – rather, the coils. The thick wire is the main tuning coil, which accepts 0.4~0.6 Amps, the small coil around the magnetic center pole is the FM modulation coil. This is for much lower currently but high bandwidth modulation. All is sealed and soaked with epoxy resin. Note the hand made labels which may explain the cost of these units if purchased new… looks like US style handwriting to me.

yig-internals-mag-coil

Well, seems that fixing this is beyond what I can do here with the tools at hand. So will need to look for a spare/used 8-13.4 GHz YIG/YTO somewhere.

A precision pendulum clock, and an even more precise time interval counter

Several years ago, I managed to get hold of a rather special piece, an electromechanical master clock, including an Invar temperature compensated precision pendulum. Such clocks were use to control various remote clocks at a train station, in large factories, or huge governmental offices, etc.

These clocks have long been superseeded by crystal oscillators, nevertheless, they are marvelous pieces of precision engineering, and it has been a long held thought of mine to measure how accurately this time-piece is performing, not only long-term (which can be easily timed by daily checks), but also short time, for each individual tic.

The pendulum has a 1/2 period of 0.75 seconds, which is quite common for such kinds of electromechanical clocks. Only the most wealthy businesses opted for a 1 m (full) pendulum, and the instrument is large enough anyway even with 3/4 lenght.

eclk-full-view

The dial is quite beautiful, and every piece appears to be hand-made. There is no date on the clock, but the age of the capacitor suggests 1920~1930 time-frame. I fully rebuild the mechanics about 3 years ago, all degreased, checked, and freshly lubricated with clock oil. There was no need for any other repair, all parts and bearings are still in good shape.

eclk-dial

The clock has a rewind mechanism that is activated once per minute, and also turns a polarity reversal switch, used to steer the remote clocks.

eclk-pickup

To get the signal out of the clock, a small light gate has been setup inside, and a 1 mm wire connected to the lower end of the pendulum (in a way to ensure virtually no movement of the wire). The wire interrupts the light gate approximate at the lowest point of the pendulum, i.e., when it has its highest speed – this is to ensure sharp edges of the signal.

eclk-setup

The setup, currently it is just a set of boards running the TIC4 and LOGGER5 software discussed earlier. A Dell OptiPlex FX160 is used to collect the data, but you can use any kind of computer that can handle RS232 input.

eclk-boards

Here, some first results – more to come. Phase is given in seconds, and horizontal axis shows the tick counts – about 115k ticks per day. The software uses narrow time gating to sort out any incorrect ticks, caused by electrical interference, or other random disturbances. There are no more than 1-2 of such events every day. The phase reconstruction algorithm also handles any missing ticks, and the measurement accuracy is not compromised if one or more tick events are not registered for any random reason.

eclk-phase-drift

Removing the linear part – not a lot of residual phase error left. Plus minus a fraction of a second a day. Now I have slightly slowed down the clock by removing a tuning weight from the pendulum.

eclk-drift-removed-residuals

As described earlier, the LOGGER5 setup also records real-time (not to a high degree of precision, just to keep track of time and day), and temperature/pressure. See earler post, LOGGER5, and TIC4.

eclk-temp

eclk-pressure

Below, and interesting feature of the data – with the re-loading of the clock every minute, there is some slight variation of the frequency. It is really not much considerable the notable “CLICK” with every re-wind, done by a large magnetic coil, actuating a lever mechanism.

minute-variation

The hourly variation, most likely, related to the travel of the minute hand – will check this later, simply by removing the hands!

hourly-variation

Some first correlation of frequency vs. pressure, but will need to collect much more data, and then correlate with pressure and temperature.

phase-res-vs-pressure

Finally, the Allan variation of the clock, determined from a few days worth of data. Short term stability is compromised by the bi-directional pick-up of the pendulum (detection is at the rising edge of the pulse, which corresponds to two different positions of the pendulum relative to the light gate – because of the discrete thickness of the wire).

eclk-allan

Perfect time: upgrade to a Motorola M12+ receiver, and new GPS antenna

For years, a Motorola UT+ GPS timing receiver has served me well as a frequency reference and source of accurate time (and location). While I primarily use a DCF77 locked 10 MHz OCXO, the GPS time is useful for various purposes, be it, to confirm that DCF77 is actually delivering the proper time.

One drawback of the Motorola UT+ is the rather large “sawtooth” error, which is caused by the quantization of the 1 pps signal derived from a 9.54 MHz clock. This results in a +-52 phase inaccuracy – which can be corrected, but only with further effort.

The later model, which is dated by now and available at low cost, the Motorola M12+, is much better in this respect, featuring a +-13 ns sawtooth, which is not a lot, and good enough for most purposes without any further corrections.

Below, some tests on an OCXO vs. GPS 1 pps pulses, for a OCXO under test (10 MHz, divided down to 100 kHz, and phase displayed in microseconds).

ocxo-vs-ut

ocxo-vs-m12-100ns

This is the small board, not a thing of beauty, but working. The only parts needed are +3.0 V and +5 V (actually using +4.4 V) voltage regulators: 3.0 V for the M12+, 4.4 V for the GPS antenna.
The 3.0 V also powders a MAX3232 TTL to RS232 converter.

m12plus-board

Also procured a second-hand GPS timing antenna – this one has a nice radome, a quadrifilar helix element, and a 26 dB amplifier to compensate any cable losses. The cable, LMR-195, features N to SMA connectors, and a considerable of PVC tape was used to protect the N-connector from the elements. Still it would be better to use some special outdoor N connectors, but, sorry, don’t have.

m12plus-antenna

m12plus-tac

A handy program to control the GPS – TAC32. Usual procedure is to carry out a location survey, which will take about 2-3 hours, and then continue in position hold/timing mode.

One drawback of my location – there is no way to get full 360 deg view, so reception is limited to the more southern satellites. But usually 6-8 satellites are in sight.

Still contemplating if it is worthwhile to put this in a larger box, together with a 10 MHz OCXO, and possibly a DS1023-50 delay line to implement a hardware sawtooth correction. Maybe a good project for winter time.

DCF77 vs. GPS time comparison: not a lot of uncertainty…

Some folks were asking about the accuracy of the DCF77 10 MHz standard described earlier, DCF77 10 MHz – which has an Piezo brand OCXO, steered by a long-time-constant PLL locked to the DCF77 77.5 kHz carrier.

But, how to assess the short and long term stability of such a ‘standard’ in practical terms? Well, short term accuracy – it will simply be that of the Piezo OCXO, and some noise injected by the power supply. Mid- and long term, the drift will be determined by the DCF77 master clock (which is dead accurate), and the propagation conditions of the long wave signal (which is by far worse).

With my location at Ludwigshafen, Germany, I’m reasonably close to the DCF77 transmitter – maybe 70 miles? So there is hope that the transmission induced effects are not all that bad.

To measure the mid and long term stability, see below two plots of the DCF77-locked phase of the Piezo OCXO, vs. the instantaneous phase of GPS, stable to 40 ns or better, and obtained from a Motorola M12+ timing receiver. Measurements were done by measuring the time interval from the GPS 1 PPS signal, to the rising edge of a 10 kHz signal – derived from the 10 MHz OCXO by a good divider (using a ADF41020 REF input – R divider routed to MUX output) by HP 5335A counter.

dcf dcf vs gps time day 57603

dcf dcf vs gps time day 57604

In short – DCF77 is tracking GPS extremely well, and the OCXO phase is stable to within a few 10 to 100 ns. In practical terms, 1 second of observation time would be well enough to calibrate any frequency standard to 1 ppm or better, by comparison with the DCF77 locked OCXO. In other words, the DCF77 locked OCXO instability appears to be dominated by the propagation of the DCF77 signal more then anything else.

ADF41020 18 GHz PLL: universal divider and PLL board

I cannot praise Analog Devices enough for the ingenious designs, and for providing parts like the ADF41020, a fully integrated 18 GHz PLL. This is actually part of a major design effort for a multi-channel frontend, here just a description of the small test board used to establish the general circuit layout and board design.

Probably interesting is also the hand-soldering of the LFCSP leadframe package, which is actually not as difficult as it seems. For soldering of the pad, there is a large via in the center, which does provide good heat-sinking and is easy to solder through the 1.2 mm board.

pll18d0 layout

Above, the layout, below, 10 boards – 14 dollars and a few weeks later.

pll18d0 pcbs

For soldering, best use 0.5 mm Ag-containing SMD solder, with Type 32 flux, which is halogen free resin flux.

pll18d0 solder

To mount the LFCSP, first apply some solder to the chip pads, but not to the center/heat sink pad. Apply some flux to the board (which is already pre-tinned; use any good SMD flux pen). Then align with a good magnifier, using some Kapton tape to hold the chip in place – leave one side exposed. Then solder, in one stroke, using a medium hot soldering tip. Reflow another time – one side done. Remove the Kapton tape, and solder the remaining 3 sides. Then stick down the chip with Kapton tape again (to avoid any remote chance of movement, in case all the solder melts during the next step). Turn around the boards, and solder through the via, with a fine solder tip.

pll18d0 via

pll18d0 adf41020 mounted

pll18d0 full board

For a test, just apply a test signal to the input, and use the “MUX” output to check for any pulses. There we go:

pll18d0 2215 pulses

These pulses aren’t quite long, so it is one of the few occasions where a scope more advanced than the 2215 Tektronix is really useful in the home shop… same pulses on a HP/Agilent/Keysight 54720A, 54713B plug-in, and 100 MHz 1:10 probe.

pll18d0 pulse out

These fast risetime pulses, and the various prescalers, dividers and good input sensitivity make the ADF41020 quite useful for any PLL and frequency counting applications.

pll18d0 2ghz in 25 khz out

2 GHz in, 25 kHz out — confirmed.

About the input sensitivity: the ADF41020 is specified over a 4 to 18 GHz range – how about lower frequencies? A quick look at the input circuit shows a 3 pF capacitor – which equals a reactance of about 53 Ohms, at 1 GHz (i.e., the capacitor and termination resistor will cut the input power available to the buffer approximately in half).

pll18d0 rf input

pll18d0 input sens

… quite useful down to 1 GHz, no problem or instability at all. Also checked the the reproducibility, for 3 devices – not a lot of scatter.

Major relocation, and numerous 230 V conversions…

Long time no post, not because there is nothing happening here, more to the opposite. Relocated from the US, East Coast, back to Germany, including the US section of my electronics shop, 40+ pieces of heavy test gear. All made it over the sea just fine, in a 20′ container. Now, changing all the fuses and converting everything to 230 VAC mains voltage. I will spare you the details, just a few impressions for some trusty HP power supplies. These actually require some re-wiring, you have to break to circuit traces, and install a wire bridge.

230v 6205c

230v 6209a

230v pcb traces

230v plugs cut off

Installing new plugs… wires properly protected.

230v plug

230v traces broken

Now just install a bridge between the middle solder points. Great that there are schematics and manuals, even for 50 year old devices!

230v schematic

230v fuse

…don’t forget to replace the fuse with one of the proper size for 230 VAC operation!